This invention relates to a catheter system for measuring blood pressure while simultaneously measuring the degree of oxygen saturation in blood. The invention is described in conjunction with measuring oxygen saturation as well as blood pressure within the cardiovascular system, although these two parameters may be measured in other systems of the body.
It is known in the prior art to provide an optically based catheter system for measuring the degree of oxygen saturation in blood of a living body while simultaneously measuring blood pressure at a site of interest. Such systems are known, for example, in the U.S. Pat. Nos. to Takayama, 3,822,695 and Mori, 3,814,081. In such systems, the two parameters, oxygen and blood pressure, are measured simultaneously employing an elongated catheter containing a bundle of optical fibers. These catheters are provided with an end hole at the distal end with the end hole being covered with a diaphragm. The diaphragm is transparent to light passing through the optical fibers in the red and infrared regions, but reflects light at shorter wavelengths. The oxygen content of blood is determined in such systems by comparing the absorption of light in the infrared region to absorption of light in the red region. Thus, carbohemoglobin and oxyhemoglobin in the blood are different in the light absorption spectrum. Both have like absorption rates for light in the infrared region, but carbohemoglobin has a much larger absorption rate for light in the red region. The percentage content of oxygen contained in the blood, that is, the ratio of oxyhemoglobin to carbohemoglobin is obtained in such systems by transmitting light in both the red and infrared regions into blood and then detecting light reflected from the blood and determining therefrom the ratio of the amount of reflected light in the two regions.
The systems described in the aforesaid patents measure the blood pressure at the site of interest with the diaphragm covered end hole. The diaphragm reflects the shorter wavelength light back into the optical fibers for transmission to an externally located meter. Since the catheter is inserted into the bloodstream of a patient, the blood pressure deflects the diaphragm, causing modulation of the light intensity so that the meter provides an indication of blood pressure.
Such catheters employing diaphragm covered end holes actually measure total pressure rather than the desired measurand; mainly, static pressure. By aligning the end hole of a catheter with the direction of blood flow, kinetic energy terms are introduced. If the catheter end hole is directed upstream, a kinetic term will be added to the pressure. If the end hole is facing downstream, the kinetic term will be subtracted from the pressure. The magnitude of the error will vary with the velocity and density of the fluid. This error will vary during the course of a cardiac cycle and will distort the shape and magnitude of a pressure wave. In the pulmonary artery, the kinetic pressure may be on the order of 10% of total pressure at rest and 50% of total pressure at a cardiac output equal to three times that at rest. The importance of the kinetic pressure error is particularly great in stenotic areas where velocities are high.
The catheters discussed above are also limited in their application to measuring pressure at a single site at a time. If pressure readings are required at different sites, then the readings must be taken at different times and the catheter must be moved so the transducer at the distal end is moved from site to site.
It is known in the prior art to provide a catheter capable of performing intravascular pressure measurements in more than one site with the readings being taken simultaneously. One such device known in the prior art is disclosed in the U.S. Pat. No. 4,543,961, to D. C. Brown, assigned to the same assignee as the present invention. In Brown, there is provided an elongated catheter having a plurality of optical fibers aligned end-to-end in the lumen of the catheter. A plurality of pressure transducers are provided along the length of the catheter with each being associated with the spacing between two aligned optical fibers. The pressure transducer includes a filter-mirror which is movable between the adjacent ends of two spaced apart fibers, the movement being in response to pressure acting against the catheter. Light is directed into the proximal end of the catheter and is transmitted by the optical fibers. At each pressure transducer, light at one wavelength only will be modulated in accordance with pressure. All other light will remain nearly unchanged. Consequently, at the proximal end, the reflected light of three different colors may be individually examined to determine the pressure exerted at each of the three sites under examination.
The multiple site pressure transducer structure in Brown serves to provide measurements of but a single blood parameter; namely, blood pressure. There is no teaching of measuring other blood parameters, such as the oxygen saturation in blood. Moreover, Brown's construction employs a plurality of optical fibers aligned end-to-end within the lumen of an elongated catheter. This results in difficulty of construction, since a typical catheter may have a diameter on the order of 0.06 inches and the optical fiber carried therein may be on the order of 400 micra. Placing a plurality of such fibers in end-to-end alignment along with associated filter-mirrors within such a catheter presents substantial difficulty in manufacture.